Continuous hot-dip galvanizing lines are known in the art. A cleaned strip of steel is heat-treated and passed from the furnace into a coating bath without being exposed to air. The coating bath contains molten zinc or zinc-aluminum (Zn—Al) alloy. As the strip emerges from the coating bath, an air knife is directed at both sides of the strip to control the weight and thickness of the coating.
After the strip enters the coating bath from the submerged furnace snout, the strip is held under the surface of the liquid metal by a submerged roll called a sink roll. Inter-metallic particles and oxides form in the bath and create undesirable substances known as dross. Dross occurs in several forms, and each form has several causative factors. The primary causes of dross are impurities in the bath (primarily iron) and temperature differentials between the molten bath, the entering substrate steel, and the sink roll equipment. Dross can form on the sink roll, causing degradation in the quality of the coated strip metal in the form of dents, resulting in defective product that fails to meet product specifications. By successfully removing the dross from a sink roll, it is possible to increase the yield and the quality of the coating process.
A sink roll assembly with accumulated dross must be replaced periodically for machining of the surface within acceptable tolerances. In zinc-aluminum continuous coating lines, replacement of a sink roll frequently takes two to four hours, and sometimes longer, during which the continuous production line is idle. When there is no dressing or scraping of the sink roll, a typical sink roll assembly in a high-speed coating line operates for three to five days, (or nine to fifteen operating shifts). The roll assembly must be disassembled, machined and reassembled, at considerable time and expense, before it can be placed back into the coating line.
In some cases, dross is removed manually by a worker manipulating a pole-mounted scraper, requiring the worker to stand directly above the pot containing molten metal 55% Zn—Al at 1100° F. For worker safety and for environmental reasons, it is desirable to avoid handling manual tools directly above liquid metal.
Mechanical scrapers have also been employed to solve the problem of dross buildup. There are two types of mechanical scrapers for cleaning the dross from sink rolls. A full-width blade is a stationary blade contacting the rotating sink roll. The blade extends the entire width of the sink roll. The full width blade wears to the profile of the sink roll over time due to the constant friction. Sink rolls are periodically removed for resurfacing, and may be machined with a crowned profile. Also if dross does appear on a sink roll despite the use of the full width mechanical scraper, the defect creates a wear spot on the scraper blade, thus permanently transferring a defect to the finished steel product.
A second type of mechanical scraper blade employs a short blade, approximately ¼ or less of the roll width. The short-blade scraper device is disposed above the molten metal bath and traverses the entire width of the sink roll by a worm drive, from which the scraper blade depends. The pressure applied by the scraper blade against the sink roll is adjusted by various means, such as by a system of weights and flotation device appended to the scraper arm to counter the weight of the blade; or by the use of a scraper blade drive unit responsive to a torque sensing device to regulate the pressure of the traversing blade. The use of a torque sensor unit in combination with a scraper blade driver is complex and expensive. The floatation device, however, is cumbersome and inflexible, requiring the operator to physically add or remove weights or floats for adjustment. Moreover, the positioning of a worm drive above a molten metal bath introduces corrosion and bending of the worm drive member in the hot environment.
Controlling the force applied to the sink roll by the scraper is critical, since the sink roll rotates by the frictional force between the steel and the sink roll as the strip passes under the sink roll. The application of excessive force may cause the sink roll to slip against the steel strip, creating scratches and other defects. By contrast, application of insufficient force may result in accumulation of dross.
Thus, there is a need for an improved sink roll scraper blade system with automatically controlled scraping pressure, and a traversing means disposed away from exposure to the molten metal bath.